Eosinophil peroxidase differs from neutrophil myeloperoxidase in its ability to bind antineutrophil cytoplasmic antibodies reactive with myeloperoxidase.

The majority of antineutrophil cytoplasmic antibodies (ANCA) in patients with systemic vasculitis (a subset of whom exhibit eosinophilia associated with systemic vasculitis) recognize two neutrophil primary cytoplasmic granule constituents, namely myeloperoxidase (MPO) and proteinase-3. As eosinophil peroxidase (EPO) shares 68% amino acid identity with neutrophil MPO, we have investigated whether serum ANCA reactive with MPO (P-ANCA) also reacts with EPO. In this study we demonstrate that P-ANCA-reactive serum binds to ethanol-fixed neutrophils but not to eosinophils in a perinuclear pattern as assessed by indirect immunofluorescence. In addition, P-ANCA reactive serum did not bind to purified EPO or alter purified EPO function as assessed in a guaiacol assay of EPO activity. These studies suggest that the antigenic epitope recognized by P-ANCA-reactive serum is not determined by the homologous 68% peroxidase sequence shared by neutrophil MPO and EPO.